1. Field of the Invention
The present invention relates to a coding method and apparatus for dividing an input audio signal into predetermined coding units such as blocks and frames and carrying out a coding processing to each of the coding units, as well as a decoding method and apparatus for decoding the coded signal.
2. Description of the Prior Art
There are known various coding methods carrying out signal compression by utilizing statistical features and human hearing sense characteristic in the time region and frequency region of an audio signal (including a sound signal and an acoustic signal). As such a coding method, there is a so-called CELP (code excited linear prediction) coding such as a VSEL (vector sum excited linear prediction) coding method and a PSI-CELP (pitch synchronous innovation--CELP) coding method which are now in a spotlight as a low bit rate audio coding method.
In a waveform coding method such as the CELP coding method, a predetermined number of samples of an input audio signal are used as a coding unit such as a block r frame, so that each block or frame of audio time axis waveform is subjected to the analysis-by-synthesis method so as to carry out a closed loop search of a optimal vector for quantization of the waveform and output an index of the vector.
A coded bit obtained by the aforementioned audio coding method of a low bit rate such as 2 kbps or 4 kbps can be widely applied as an audio in general to communication, computers, broadcasting, and the like, irrespective of particular audio information contents. Accordingly, it should be strictly protected against an error caused in a transmission path.
If an error is repeatedly caused in a transmission path, audio decoding is accompanied by a long time of missing sound, deteriorating the audio quality.